The present invention relates generally to methods and related apparatus for enhancing heat transfer to or from a fluid flowing cross-wise in contact with the outer thermally-conductive shells of a plurality of axially-oriented heat exchange conduits capable of acting as heat sources or heat sinks. By channeling cross-wise fluid flow, flowing generally orthogonal to the axes of the heat exchange conduits and contouring it upstream, downstream and/or around or alongside the heat exchange conduits utilizing slotted or apertured plates, baffles or surrounding sleeve-like elements, a surprisingly more effective and efficient heat transfer between the flowing fluid and the thermally-conductive surface is realized.
It is well known to heat or cool process fluids, which may be liquids or gases, by flowing them into contact with a thermal-transfer surface that is maintained at a temperature which is different from that of the upstream process fluid thereby resulting in heat transfer either to or from the process fluid (depending on whether the thermal-transfer surface is maintained at a higher or lower temperature than the fluid). In one familiar version of this technology, the thermal-transfer surface that acts as a heat source or heat sink is the exterior of a thermally-conductive shell of a thermal-transfer tube or pipe, for example, which is heated or cooled by means of a liquid flowing axially through the interior of the tube or pipe. In a variation of this technology, heat may be supplied directly inside a heat exchange conduit by means of flameless combustion of fuel gas (such as hydrogen or a hydrocarbon) as taught, for example, by U.S. Pat. Nos. 5,255,742 and 5,404,952, which are incorporated herein by reference.
It is also known in the art to flow a process fluid axially along a thermal-transfer surface, either concurrently or counter-currently relative to the direction of liquid flow inside the thermal-transfer tube, or to crossflow the process fluid relative to the axis of the thermal-transfer tube, or some combination of the two. Typical applications of heat transfer between crossflowing fluid and heat exchanging conduits are found in air coolers, economizers associated with fired heaters or furnaces, and in shell and tube exchangers. Various types of so-called radial or axial/radial flow reactor designs are known for various applications whereby at least a part of a fluid process stream moves, at some point, through the reactor in a radial, crossflow direction (i.e., inward-to-out or outward-to-in), as contrasted with the more familiar axial flow (i.e., end-to-end) reactor designs. Examples of reactor designs embodying at least in part a radial, crossflow of process fluid relative to a plurality of axially-disposed heat-transfer tubes are shown in U.S. Pat. Nos. 4,230,669; 4,321,234; 4,594,227; 4,714,592; 4,909,808; 5,250,270; and 5,585,074, each of which is incorporated herein by reference.
Although crossflow contact of a process fluid with a heat-transfer surface can be an attractive option for many applications, the utility of crossflow contact for industrial applications has been limited by certain heat transfer inefficiencies which have been experienced in practice. Typically in crossflow designs, a given portion of the process fluid is in contact with the heat-transfer surface for a shorter time than with a comparable axial flow design. In addition, the contact between the crossflowing process fluid and the heat-transfer surface is uneven due to process fluid separation and recirculation. Short surface contact time, uneven contact, and limited fluid mixing can lead to inefficient, insufficient, and/or non-uniform thermal energy transfer.
Thus, in an article entitled xe2x80x9cImpingement heat transfer at a circular cylinder due to an offset of non-offset slot jet,xe2x80x9d appearing in Int. J. Heat Mass Transfer., vol. 27, no. 12, pp. 2297-2306(1984), the authors Sparrow and Alhomoud report experimental efforts to vary the heat transfer coefficients associated with crossflow of a process gas relative to a heat-transfer tube by positioning a slotted surface some distance upstream of the heat-transfer tube to create a gas jet. Sparrow and Alhomoud varied the width of the jet-inducing slot, the distance between the slot and the tube, the Reynolds number (degree of fluid turbulence), and whether the slot jet was aligned with or offset from the tube. The authors concluded that the heat transfer coefficient increased with slot width and Reynolds number, but decreased with slot-to-tube separation distance and offset.
Because the Sparrow and Alhomoud study concluded that the heat transfer coefficient increased with slot width, the general utility of an upstream slot to increase heat transfer is at best ambiguous based on these results. It can only be concluded that, in the experimental design used by Sparrow and Alhomoud, a relatively wider slot led to a higher heat transfer coefficient than a relatively narrower slot, and no upstream slot at all might yield the highest value. No testing was performed utilizing a plurality of heat-transfer tubes, or using upstream and downstream pairs, or around or alongside flow constriction means to preferentially contour crossflow fluid paths in contact with the outer surface of each of a plurality of heat-transfer tubes, and no reasonable extrapolations can be made to such very different alternative designs and configurations based on the extremely limited data presented.
These and other drawbacks with and limitations of the prior art crossflow heat exchanged designs are overcome in whole or in part with the enhanced crossflow heat transfer methods and designs of this invention.
Accordingly, a principal object of this invention is to provide methods and designs for enhanced crossflow heat transfer between a process fluid and a heat-transfer surface.
It is a general object of this invention to provide methods and designs for specially directing and shaping fluid crossflow paths in contact with one or more heat-transfer surfaces so as to enhance heat transfer between the fluid and the heat-transfer surfaces.
A specific object of this invention is to provide fluid flow-constriction means upstream, downstream and/or around or alongside a heat-transfer surface so as to preferentially contour a process fluid stream flowing cross-wise past the heat-transfer surface to enhance heat transfer between the fluid stream and the heat-transfer surface.
A further specific object of this invention is to provide curved or flat apertured plates or apertured sleeves disposed relative to each conduit in an array of heat exchange conduits so as to preferentially contour the flow path of the fluid stream flowing cross-wise past the outside of each of the conduits to realize improved heat transfer.
Still another object of this invention is to provide heat-transfer conduit arrays of varying sizes and configurations wherein each conduit of the array is associated with its own fluid flow-constriction means upstream, downstream and/or around or alongside of the conduit so as to preferentially contour the portion of the fluid stream flowing cross-wise past the outside of the conduit to realize improved heat transfer.
Other objects and advantages of the present invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises, but is not limited to, the methods and related apparatus, involving the several steps and the various components, and the relation and order of one or more such steps and components with respect to each of the others, as exemplified by the following description and the accompanying drawings. Various modifications of and variations on the method and apparatus as herein described will be apparent to those skilled in the art, and all such modifications and variations are considered within the scope of the invention.
In the present invention, a baffle structure comprising at least a paired set of fluid flow constructors is utilized to preferentially contour the flow path of a process fluid flowing cross-wise, or substantially cross-wise, in contact with a heat-transfer surface in order to enhance heat transfer between the fluid and the surface. The apparatus is designed so as to substantially restrict the bypassing of fluid flow such that a predominant portion of the process fluid is forced to flow past the heat-transfer surface. The heat-transfer surface will typically be one or a configured array of heat exchange conduits, oriented to have parallel axes disposed in an axial direction which is generally orthogonal to the direction of fluid flow, and having a thermally-conductive shell. The exterior surface of the shell of each such conduit is maintained at a temperature different from that of the upstream process fluid so that thermal energy is transferred to or from the process fluid by means of conduction, convection, radiation or some combination thereof, as the fluid flows past and contacts the exterior surfaces of the heat exchange conduits.
The heat exchange conduits or ducts of this invention may broadly comprise tubes, pipes, or any other enclosures with heat sources or heat sinks. The exterior surfaces of the heat exchange conduits may be bare or, as discussed below, may be finned or any combination of the two. The cross-section of the conduits or ducts may be circular, elliptical, or any other closed shapes. Where a plurality of such heat exchange conduits are used, they will typically be arrayed in some predetermined configuration such as in a triangular array, a square array, a circular array, an annular array, or other such patterns depending on design choice and/or the requirements of a particular application. Relative to the direction of fluid flow, adjacent conduits may be aligned, staggered or otherwise positioned, again depending on design choice and/or application requirements.
The size of the heat exchange conduits will be dictated, at least in part, by process requirements for the rate of heat transfer. In general, conduits having larger cross-sections (for any given conduit geometry) will provide larger surface areas and therefore more heat transfer capacity. Fin elements, baffles or other heat-transfer enhancing structures may be provided on the outside surface of some or all of the heat exchange conduits to further increase surface area and improve heat transfer characteristics. A preferred embodiment utilizes closely spaced circumferential fins applied in a spiral along the exterior length of the conduit. This arrangement increases the heat-transfer surface area exposed to the crossflow without impeding the flow. It will be understood that the nature and flowrate of the process fluid, and the desired temperature change in the fluid between upstream of the heat exchange conduits and downstream of the conduits, will also affect these design choices.
The fluid flow constriction means for contouring the cross-wise flow of the process fluid may comprise inlets, outlets and openings of various shapes and sizes in baffle structures located upstream, downstream and/or around or alongside the heat exchange conduits. In a further preferred embodiment, each heat exchange conduit has its own associated pair of upstream and downstream fluid flow constrictors or its own around or alongside flow constrictors as described below. The apertured baffle structures which function as fluid flow constriction means may comprise plates, sleeves or other baffles which comprise substantially flat surfaces, or curved surfaces, or a combination of flat and curved surfaces. Apertured structures of this type positioned in pairs upstream and downstream of an array of heat exchange conduits have been found to enhance heat transfer by a factor of about one and one-half to about two times. In a particularly advantageous embodiment for certain applications, the fluid flow constriction structure is a larger, generally concentric sleeve-like structure at least partially surrounding each conduit in an array of tubular heat exchange conduits, each such sleeve structure having apertures upstream and downstream of the centrally-located heat exchange tube. Apertured sleeves of this type at least partially surrounding individual heat exchange conduits in an array of such conduits have been found to enhance heat transfer by a factor of about five times or more.
The apertures in the fluid flow constriction structure preferably comprise any combination of perforated holes or axial slots (i.e., elongated apertures having a longer axis generally parallel to the axial orientation of the heat exchange conduits). The holes or slots in different portions of the apparatus may be the same or differ in curvature, size and shape. The edges around the inlets and outlets may be straight, rounded, jagged, or some combination thereof.
The fluid flow constriction structure is preferably positioned relative to an associated heat exchange conduit such that the distance between the centerline of an upstream or downstream aperture and the associated heat exchange conduit centroid ranges from about 0 to about 2.0, preferably from about 0.50 to about 1.00, times the outer diameter (or largest cross-sectional dimension of a non-circular conduit) of the conduit. In any case, the spacing between aperture and conduit must be sufficiently close to realize substantially enhanced heat transfer. The width (shortest side) of an elongated flow constriction aperture or the diameter of a generally circular hole constriction aperture may preferably range from about 0.02 to about 1.5, preferably from about 0.05 to about 0.25, times the outside diameter (or largest cross-sectional dimension of a non-circular conduit) of the conduit. The fluid flow constriction structure is preferably positioned relative to an associated heat exchange conduit such that the offset between the center of the aperture and the centroid of the heat exchange conduit ranges from 0 to 0.5, preferably 0, times the outside diameter (or largest cross-sectional dimension of a non-circular conduit) of the conduit.
The enhanced crossflow heat exchange apparatus of this invention enhances heat transfer between the crossflowing fluid and the plurality of heat exchange conduits by one or more of the following mechanisms: (a) increasing the fluid velocity around the heat exchange conduits; (b) preferentially directing the fluid to closely follow the outer surface of the heat exchange conduits; (c) restricting the fluid from flowing into or through areas that are distant from the outer surface of a heat exchange conduit; (d) reducing xe2x80x9cdeadxe2x80x9d regions and flow recirculation around heat exchange conduits; (e) enhancing fluid turbulence; and (f) enhancing mixing between colder and hotter portions of the fluid.